Method of producing chromium



Nov. 17, 1959 Filed May 31, 1956 R. B. EATON ETAL 2,913,333

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METHOD OF PRODUCING CHROMIUM Filed May 31, 1956 4 Sheets-Sheet 2 RUSSELL B. EATON WILLIAM S. WARTEL Nov. 17, 1959 Filed May 31, 1956 R. B. EATON ETAL 2,913,333

METHOD OF PRODUCING CHROMIUM 4 Sheets-Sheet 3 swam bow RUSSELL B. EATON WILLIAM S. WARTEL Nov. 17, 1959 R. B. EATON ETAL ,91 3

METHOD OF PRODUCING CHROMIUM Filed May 31, 1956 4 Sheets-Sheet 4 7 vi avwwvtou/ RUSSELL B. EATON WlLLlAM S. WARTEL FIG-4 W United States Patent METHOD OF PRODUCHIG CHROMIUM Russell B. Eaton and William S. Wartel, Wilmington, DeL, assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application May 31, 1956, Serial No. 588,359

12 Claims. (Cl. 75-845) This invention relates to the production of ductile chromium, and more particularly to a novel process for producing free-flowing, crystalline chromium powder, essentially all of which is in dendritic crystal form.

Metallic chromium is usually obtained from its ore by the reduction of the ore with aluminum according to the well known thermite process. Chromium produced in this manner, however, is not of the highest purity, and it lacks ductility. For this reason, additional methods have been developed in the continuing search to find better means for producing ductile chromium. Most of these methods have been based upon the reduction of the halides of chromium. For example, chromium has been produced in sponge form by the reduction of chromic chloride with magnesium.

It is an object of this invention to provide a novel step-wise process for reducing chromium trichloride to a free-flowing, metallic chromium powder which exhibits a dendritic crystal structure. This dendritic crystal chromium powder is high-purity chromium with excellent ductility, and it is uniquely adaptable to powder metallugrical techniques not previously possible with chromium. Further objects and advantages of the invention will be evident from the ensuing description.

Fig. 1 is a schematic drawing of one form of apparatus which can be used to carry out this invention. Figs. 2, 3 and 4 show chromium particles obtained by the process of this invention at 100, 1000 and 7000 diameters magnification, respectively.

The objects of this invention are attained by our process for the production of ductile chromium in a freeflowing powder exhibiting a dendritic crystal structure. This process is step-Wise, and the first step comprises reacting a chromium trihalide with hydrogen within a closed reaction zone and in the presence of a salt selected from the group consisting of alkali metal halides and alkaline earth metal halides and mixtures thereof. The reaction temperature is kept above the melting point of the chromium halide-salt composition and below its boiling point, and the reaction is carried out until all of the chromium trihalide has reacted with hydrogen and a chromium dihalide-salt composition is formed. This can be accomplished by continuing the flow of hydrogen until a small portion of the chromium halide is reduced to chromium or a sample of the chromium dihalide salt composition fails to indicate chromic ions. upon analysis. Still further objects are obtained by maintaining the chromium dihalide-salt composition thus formed in the molten state in a closed reaction zone containing an inert atmosphere and then incorporating with the molten composition at least a stoichiometric quantity of a reductant metal, such as magnesium, while agitation is being eifected. Under these conditions a free-flowing, dendritic crystal chromium powder consisting of high-purity chromium of excellent ductility is obtained. This chromium may be separated from the salt by conventional separation means such as draining or washing.

in a more specific and preferred embodiment, the invention comprises reducing high-purity, anhydrous chromium trichloride to chromium dichloride in the presence of an alkali metal halide, such as sodium chloride, or a mixture of sodium chloride and potassium chloride, in a reaction zone which is maintained at a temperature above the melting point and below the boiling point of the chromium chloride-salt composition. The chromium dichloride-sodium chloride composition thus formed is then effectively agitated with an active reducing metal such as magnesium to obtain the dendritic chromium. The chromium is thereafter separated from the mixture of sodium chloride carrier and magnesium chloride by-product and then it is usually further purified by leaching in an aqueous acidic medium (e.g., dilute nitric acid).

The free-flowing chromium obtained is considered to be unique in that at 7000 diameters magnification there is only evidence of a dendritic crystal structure. It is believed that it is this characteristic crystal form which makes this metallic chromium especially suitable to powder metallurgical techniques.

Chromium trichloride is preferred as a starting halide since it is most readily available. The chromium trichloride or other chromium trihalide is usually incorporated with the alkali and/or alkaline earth metal salt in such amounts that it constitutes from 10-60% by weight of the chromium trichloride-carrier salt charge. The preferred amount of chromium trichloride constitutes from 20-30% by weight of the charge. The alkali metal salt or alkaline earth metal salt may be either a specific compound or a mixture of the specific compounds which are included within the class of alkali metal and alkaline earth metal halides. Specific materials which may be used include various halides of alkali metals and alkaline earth metals as exemplified by sodium chloride, lithium chloride, potassium chloride, calcium chloride, magnesium chloride, and barium chloride.

The upper limit on the amount of hydrogen introduced into the chromium trichloride-carrier salt charge is not critical. For economical reasons, it should be kept below about ten times the stoichiometric amount, and it is preferred to use an amount which is only slightly in excess of the stoichiometric quantity so as to insure the availability of at least a stoichiometric amount for reaction. Stoichiometric amounts are based upon the reduction of CrCl to CrCl according to the following equation:

The temperature for the hydrogen reduction should be sufiiciently high to keep some of the salt composition in the liquid state. The upper temperature limit is not particularly critical, but for practical purposes it should be below the boiling point of the end product salt composition. Preferred temperatures are usually above about 500 C., and more preferably between 700 C. and 850 C. Since the end product chromium dichloride-salt composition is at an elevated temperature immediately after the hydrogen reduction, it is ideally suited for further reduction to elemental metal. This is accomplished by the addition of a reducing metal, such as magnesium, to the molten chromium dichloride-salt composition in at least a stoichiometric amount to reduce the chromium dichloride to the metal. The magnesium addition is accompanied by effective agitation, and an inert atmosphere is maintained within the reactor to prevent contamination. The reaction zone is maintained above the melting point of the salt composition and below the boiling point of the reducing metal, and preferably between 700 C. and 850 C. The amount of reducing metal is from one to ten times stoichiometric, and preferably slightly in excess of stoichiometric, to insure good yields. The abovedescribed operation has been found to produce a homo genous product which can be commingled with conven- 3 tional reducing agents to give high yields of high-purity ductile chromium powder.

Reference is made to Fig. 1 in the accompanying schematic drawing which shows' the process of this invention in operation. Corrosion-resistant metal'reacti'on vessel 1 is mounted in conjunction with a suitable furnace 19. Inside vessel 1 is a chlorine and hydrogen chloride resistant liner 2, preferably fabricated from non-porous graphite. A non-porous graphite cover 3 is provided for the vessel, and this cover has sealed openings for gas inlet 4 and gas outlet 5 which are also constructed of non-porous graphite. At the bottom of liner 2 is a discharge port 6 and valve 7 for releasing the reactors charge to vessel 3 which is situated underneath vessel 1. Below discharge port -6 is a stainless steel filter 9 of about 5 micron pore size for removing any oxide or'other impurities from the chromium dichloride-salt composition before it is reduced to metal. Reaction vessel 8 is enclosed by furnace 20. This vessel is constructed of stainless steel, and it contains a liner '10 of mild steel or chromium which is corrosion-resistant to the reactants and the products of the reaction. Vessel 8 is provided with a stainless steel top 11 which contains sealed openings for conduit 12 which is in communication with upper vessel 1 and also for gas inlet 13, agitating device 14, reductant metal inlet 15 and gas outlet 16. These elements are preferably constructed of mild steel or chromium. The apparatus thus described is, of course, a sealed system so as to avoid contamination from the atmosphere. In reaction vessel 1 there is placed a charge of anhydrous chromium trichloride and salt carrier as designated by numeral 17 in the accompanying drawing. The reactor is then purged with a chlorinating gas, preferably a mixture of chlorine andcarbon tetrachloride, to chlorinate any residual oxide that might be present and to drive oif moisture as the temperature is raised to above the melt ing point of the salt mixture, preferably between 650 C.-750 C. The oxide removing gas is then flushed from the system with a suitable purge gas; e.g., argon or HCl. This is followed by the introduction of hydrogen, which has been carefully freed of impurities, through inlet 4, and reduction is carried on until all of the chromic chlo ride is reduced to chromous chloride, and preferably until a small amount of chromium is obtained. Reduction of a small amount of the dichloride to the metallic state will thus serve two important functions: first, it will insure the complete reduction of the trichloride to the lower-valent state; and secondly, this small amount of chromium metal will react with impurities to form oxides which will be caught by filter 9. Alternatively, a small amount of metallic chromium may be added to the reduced chromium dichloride-salt composition to serve these purposes. 9 When the salt carrier is thus freed of chromium in the trivalent state, valve 7 is opened, and the purified salt mixture is passed through filter 9 to vessel 8 which has been previously purged free of air and cleaned of scale or any other deleterious material which could affect the purity of the product. ,The temperature is maintained within the same range as was used in vessel 1, and agitation .of the molten salt is effected through activation of the stirrer assembly which may be supplemented by continuing to bubble inert gas through inlet 13. While the products and to recover free-flowing particles of ductile chromium of greater than 99.9% purity possessing .a dendritic crystal structure. Dendritic crystals are, as the name implies, branched crystals, and are referred to thus in metallogr-aphy, as opposed to, for example, spherical crystals. In this inventionthis dendritic form is considered unique, and the crystalline dimensions usually exhibit a length-to-widith ratio of at least 10 to 1.

The following examples will serve to illustrate the invention in detail: 7

Example 1 The graphite reactor liner 2 of the apparatus shown in the drawing was charged with a dry mixture of 300 g. of K01 and 300 g. of NaCl and 240 g. of C1Cl The cover was secured; and during the heat-up to 750 C., chlorine saturated at room temperature with carbon tetrachloride was passed through the melt via inlet 4 at 5-10 c.f.h. to chlorinate any residual oxides that might be present. This flow was continued for one hour after water vapor from absorbed water in the reactants ceased issuing from the reactor. The reactor was then cleared of the Cl CCl with a five minute I-ICl purge. In the same flow-rate range, purified hydrogen was next bubbled through the salts until the reduction of chromic chloride to chromous chloride was complete and a small amount of metal formed. The amount of H used was about 10 times stoichiometric. Valve 7 was then disengaged, and the melt was filtered through a mild steel plate 9 of 5 micron pore size into the reaction vessel 8, which had been previously heated to 750 C. and purged with hydrogen. Fine shot magnesium, 2() mesh, in total quantity of 38 g. was then fed slowly from a purged container into the reactor over a period of ten minutes, during which time agitation was being carried out. The resulting-slurry of chromium particles in molten magnesium chloride and alkali salts was allowed to cool under hydrogen, and in this run was handled without drainage of by-products.

' The metal product was freed of by-product salt with several 5% nitric acid washes. Chemical analysis proved the product to exceed 99.9% purity with a distribution of the crystalline chromium according to US. Sstandard Sieve No. as follows:

charge is being agitated in this manner, magnesium, prefby cold compaction. Bars of chromium 2"'x A? x A Mesh size: Percent weight +60 4.0 -60 +270 43.4 270 which did not clearly show the dendritic structure at x were then magnified to IOOOX, and the results shown in Figure 3 were obtained. This magnification clearly establishes the dendritic structure in the particle at the left-hand side of the figure, while the particle in' the upper right-hand corner needed further magnification. Figure 4 is a 7000 diameter magnification of a portion of a particle which exhibits the pattern shown in the upper right-hand corner of Figure 3. This high degree 'of magnification by an electron microscope clearly shows that the dendritic structure is characteristic of the particles of chromium which are obtained in this invention.

The-following table shows the ability of the chromium powder of this invention to be densified into metal bars were prepared in a single-actionidie; the densities of the bars were then determined, and the results expressed in terms of the percent of theoretical density. Electrolytic chromium which was obtained from a commercial source was also cold compacted in the same manner.

From the above table, it will be seen that 25 tons of pressure on the chromium of this. invention resulted in a bar which had a greater density than a similar bar formed fromelectrolytic chromium at 100 tons pressure. .It will.

also be seen that at 100 tons pressure the density of the bar formed approached the theoretical density of chromium. In addition to this density increase, the bars made from the chromium of this invention have exceptional strength and can be readily handled and even machined prior to sintering. On the other hand, the electrolytic chromium was brittle, and the bar could be easily powdered by scraping with the fingernail. The high green density of the products of this invention improves the sin tering characteristics with the approach to theoretical density being accomplished at lower temperatures and within a shorter time than is possible with any other available chromium powder. Sintered rods made from the chromium powder of this invention are ductile when tension is applied. The chromium of this invention was also fabricated into a sheet by direct cold rolling of the powder, followed by a sintering operation. Such sheet was then fabricated into a liner for a reaction vessel.

Example II The reaction vessel 1 was charged with 500 g. of NaCl and 100 g. CrCl (99.7%) and following the positioning of the lid, the mixture was treated with a chlorine-carbon tetrachloride purge at 750 C. The assembly was then flushed with H01, and the CrCl was next reduced to CrCl with hydrogen, according to the procedure of Example Ia clear melt resulting upon completion of reduction. The mixture was filtered into reaction vessel 8, which had been preheated to 750 C. and purged free of air or any other deleterious material. Thirty grams of pressed pellets of purified sodium were fed from a purged container into the reactor over a period of 5-10 minutes during which time agitation was being carried out. The resultant metal-salt mixture was allowed to cool and freed from by-product salt with 5% HNO washes. The yield was 99.9% pure chromium in a free-flowing crystalline form, and it was obtained in excess of 85% of the theoretical yield.

Example Ill To permit continuous operation of the process, the apparatus shown in the drawing was modified so that reaction vessels 1 and 8 were side-by-side in separate furnace installations with the filter located between the assemblies, and the molten salt could be transferred from vessel 1 to vessel 8 by maintaining a higher gas pressure above the liquid in vessel 1. Reaction vessel 1 was charged with 500 g. of NaCl and 100 g. of CrCl and following the positioning of the lid, the mixture was treated with a chlorine-carbon tetrachloride purge at 750 C. The assembly was flushed with H61 and the CrCl was next reduced to CrCl with hydrogen according to the procedure of Example Ia clear melt resulting upon completion of reduction. The CrCl -salt composition free of CrCl was removed to reaction vessel 8 which had been preheated to 750 C. and purged free of air or any other deleterious material. Thirty (30) grams of pressed pellets of purified sodium were fed from a purged hopper into the reactor over a period of 5-10 minutes during which time agitation was being carried out. The chromium metal plus salt mixture thus formed in vessel 8 was drained into a purged container via valve 18 and freed from by-products by vacuum distillation. While production of the metal was being carried out in vessel 8, vessel 1 was producing additional chromium dichloridesalt mixture so that the operation was running on a continuous basis. About 1000 g. of CrCl was reduced to metal of 99.9% purity with a. yield exceeding of theoretical.

In the process of this invention, it is essential that all of the chromium trichloride is reduced by the hydrogen before the reducing metal is used to convert the chromium dichloride-salt composition to metallic chromium. Operation in this manner insures consistent production of the unique form of chromium which has been previously described.

The reactants, the salt carrier, the purging gases, and the inert gases should be substantially free of deleterious impurities, preferably above 99.9% purity. In the specific examples, a chlorinating gas mixture of chlorine and carbon tetrachloride was flowed through the system and also through .the chromium trichloridc-salt mixture to help accomplish this objective. This flow of gas will sweep out moisture, and it will serve to chlorinate any residual chromium oxide and remove the oxygen as a. gaseous product. Other contaminating metal oxides such as iron, titanium, zirconium and aluminum, will be chlorinated in the same manner, and the deleterious oxygen will be carried out in the flowing gas. stream. The more volatile metal chlorides thus formed will also be swept out in this manner while high-melting insoluble impurity materials will be removed by the filter between the reaction vessels. The chromium trichloride used in the example was the best commercially available anhydrous chromium trichloride, and this chlorinecarbon tetrachloride gas flow results in an even higher purity.

The normal atmosphere will, of course, cause contamination in the process of this invention; and for this reason, the reaction system is closed. In the hydrogen reduction step, excess hydrogen and by-product hydro gen chloride which bubbles through the molten salt will form an inert atmosphere for the reaction zone. In the second reduction step, wherein the chromium metal is formed, the inert atmosphere may be hydrogen, argon, helium or other well known inert gas. It is also contemplated that salt vapors from the carrier salt as well as those from the by-products of the reaction could be maintained above the charge to insure non-contamination. One skilled in the art will also understand that the chromium metal product should be protected by an inert atmosphere while it is at elevated temperatures.

The apparatus shown in the accompanying drawing is for purpose of illustration, and one skilled in the art could offer modifications or substitutes for this apparatus. A graphite-lined reaction vessel is preferred; however, other corrosion-resistant materials such as silica could also be used. In the second reaction vessel a chromium liner is preferred, since it will not introduce impurities into the chromium produced. However, a mild steel liner will also be suitable in this step of the reaction.

While the metal reducing agents disclosed in the specific examples are magnesium and sodium, other reducing metals which may be used include lithium, rubidium, cesium, potassium, calcium, barium, strontium, zinc, aluminum and alloys containing these metals.

The present invention offers several distinct advantages, chief among these being that it produces a highpurity ductile chromium of dendritic crystal structure in high yields. This type of chromium is considered to be a unique product which is not obtainable by any prior art processes. This invention also has the advantage of being adaptable to continuous operation and relatively simple and inexpensive equipment.

:We claim: a 7

1. Aprocess for the production of ductile chromium above the melting point of the chromium halide-salt com position and below its boiling point, said reaction being carried out until all of the chromium trihalide has re acted with hydrogen and a chromium dihalide-salt composition is formed, maintaining the above-recited reaction temperatures and a closed reaction zone and incorporating at least a stoichiometric amount of a reductant metal selected from the group consisting of Mg, Na, Li, Rb, Cs, K, Ca, Ba, Sr, Zn, Al, and alloys consisting of these metals into said chromium dihalide-salt composition while agitation-is being. effected, recovering a molten salt composition containing free-flowing particles of ductile chromium metal, and separating said metal from said salt composition.

2. The process of claim 1 in which the chromium trihalide is chromium trichloride.

3. The process of claim 2 in which the salt is an alkali metal chloride. I

4. A process for the production of ductile chromium in free-flowing crystalline form which comprises reacting chromium trichloride with hydrogen at temperatures ranging from about 700 C.850 C. within a closed reaction zone and in the presence of an alkali metal chloride, saidchromium trichloride constituting from -60% by Weight of the chromium trichloride-carrier salt charge, and the hydrogen supply to said reaction zone being about 1-10 stoichiometric amounts based on the reduction of the chromium trichloride to chromium dichloride, the reaction being carried out until all of the chromium trichloride has reacted with hydrogen and a chromium dichloride-salt composition is formed, maintaining a closed system and transferring said chromium dichloride-salt composition to a second reaction zone and at temperatures ranging from about 700 C. to 850 C., gradually adding from 1 to 10 stoichiometric amounts of a reducing metal selected from the group consisting of Mg, Na, Li, Rb, Cs, K, Ca, Ba, Sr, Zn, Al, and alloys consisting of these metals while agitation is being efiected, recovering a molten salt composition containing free-flowing particles of ductile chromium metal, and separating said metalfrom said salt composition.

5. The process of claim 4 in which the alkali metal chloride is sodium chloride. 7

6. The process of claim 4 in which the alkali metal chloride is a mixture of potassium chloride and sodium chloride.

7. A process for the production of ductile chromium in free-flowing crystalline form which comprises reacting chromium trichloride with hydrogen at temperatures ranging from 700 C.850 C. within a closed reaction zone and in the presence of an alkali metal chloride, said chromium trichloride constituting from 2030% byweight of the chromium trichloride-carrier salt charge, and the hydrogen supply to said reaction zone being about 10 stoichiometric amounts based on the reduction of the chromium trichloride to chromium dichloride, the reaction being carried out until all of the chromium trichloride has reacted with hydrogen and a chromium dichloride-salt composition is formed, maintaining a closed system and transferring said chromium dichloride-salt composition to a second reaction zone at temperatures ranging from 700 C. to 850 C., gradually adding in an amount slightly in excess of stoichiometric a reducing metalselected from the group consisting of Mg, Na, Li, Rb, Cs, K, Ca, Ba, Sr, Zn, Al, and alloys consisting of these. metals while agitation is being effected, recovering a molten salt composition containing free-flowing particles of ductile chromium metal, and separating said metal from said salt composition.

8. A process for the production of ductile chromium in free-flowing crystalline form which comprises charging a closed reaction zone with a chromium trichloride-carrier salt charge, said chromium trichloride constituting 10- 60% by weight of said charge and said carrier salt being an alkali metal chloride, raising the temperature of saidcharge to between 700 C.-850 C., and flowing a chlorine-carbon tetrachloride gas mixture through said charge and said reaction zone to remove deleterious impurities, then introducing l-l0 stoichiometric quantities of hydrogen" into said charge to reduce all of the chro-- mium trichloride and form a chromium dichloride salt composition, maintaining a closed system and transferring said chromium dichloride salt composition to a second reaction zone, which has been previously purged with hydrogen, at temperatures ranging from 700 C. to 850 C., gradually adding from 1-to l0 stoichiometric amounts of a reductant metal selected from the group consisting of Mg, Na, Li, Rb, Cs, K, Ca, Ba, Sr, Zn, Al, and alloys consisting of these metals while agitation is being eiiected, recovering a molten salt composition containing freeflowingparticles of ductile chromium metal and separating said metal from said salt composition.

'9; Theprocess of claim 8 in which the chromium trichloride constitutes 20-30% by weight of the chromium trichloride-carrier salt charge.

10. The process of claim 9 in which the alkali metal chloride is sodium chloride.

11. The process of claim 9 in which the alkali chloride is a mixture of potassium chloride and sodium chloride.

12. A process for the production of ductile chromium in free-flowing crystalline form which comprises introducinghydrogen into a chromium trichloride-carrier salt composition in a reaction zone at temperatures above the melting point of the chromium chloride-salt composition and below its boiling point, said chromium trichloride constituting from 1060% by weight of said chromium trichloride-carrier salt composition, said introduction of hydrogen being continued until all of the chromium trichloride has reacted with hydrogen and a chromium dichloride-salt composition isformed, main taining the above-recited reaction temperatures and a closed reaction zone and incorporating at least a stoichiometric amount of a reductant metal selected from the group consisting of Mg, Na, Li, Rb, Cs, K, Ca, Ba, Sr, Zn, Al, and alloys consisting of these metals into said chromium dichloride-salt composition while agitation is being effected, recovering a molten salt composition containing free-flowing particles of ductile chromium metal, and separating said metal from said salt composition.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,854 Kroll June 25, 1940 2,678,879 Nuesch et a1. May 18, 1954 FOREIGN PATENTS 207,104- Switzerland Dec. 16, 1939 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 11, page 136. Published 1931 by Longmans, Green & Co., New York.

New -nevi. 

1. A PROCESS FOR THE PRODUCTION OF DUCTILE CHROMIUM IN FREE-FLOWING CRYSTALLINE FORM WHICH COMPRISES REACTING A CHROMIUM TRIHALIDE WITH AT LEAST A STOICHIOMETRIC AMOUNT OF HYDRGEN WITHIN A CLOSED REACTION ZONE AND IN THE PRESENCE OF A SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HALIDES AND ALKALINE EARTH METAL HALIDES AND MIXTURES THEREOF, THE REACTURE TEMPERATURE BEING ABOVE THE MELTING POINT OF THE CHROMIUM HALIDE-SALT COMPOSITION AND BELOW ITS BOILING POINT, SAID REACTION BEING CARRIED OUT UNTIL ALL OF THE CHROMIUM TRIHALIDE HAS REACTED WITH HYDROGEN AND A CHROMIUM DIHALIDE-SALT COMPOSITION IS FORMED, MAINTAINING THE ABOVE-RECITED REACTION TEMPERATURES AND A CLOSED REACTION ZONE AND INCORPORATING AT LEAST A STOICHIOMETIC AMOUNT OF A REDUCTANT METAL 